1. Field of the Invention
This invention relates to a sheet guide device of an image forming apparatus such as a copying machine, a facsimile, a printer, or the like, and more particularly to a sheet guide device which can guide a sheet with its widthwise position adjusted and convey the sheet without forming wrinkles in the sheet.
2. Discussion of the Related Art
Recently, with the widespread use of digital copying machines and printers, demands for improvement in the quality of image reproduction have been increasing and, for example, a precise positioning of an image on a sheet is particularly demanded. Additionally, with increasing environmental sensitivity among users, a duplex unit for two-sided copying has become important. In an image forming apparatus with a duplex unit, the sheet conveying path is longer than in an image forming apparatus for an ordinary one-sided copy, because a sheet carrying an image on one side of the sheet needs to be reversed for receiving another image on the other side of the sheet. Because of the long sheet conveying path, it is relatively difficult to prevent a sheet from being skewed. When a sheet is skewed, image quality on two sides of the sheet becomes uneven. In order to avoid sheet skew and the resulting lateral deviation of sheet positioning in the duplex unit, the duplex unit has been generally configured such that the sheet is temporarily stacked in an intermediate tray and is aligned by being jogged with side fence joggers.
FIGS. 1(A) and 1(B) illustrate a background duplex unit having a sheet positioning adjustment device. The duplex unit includes a reversible reverse roller 201, an intermediate tray 202, and a pair of side fences 203. The pair of side fences 203 moves in the direction indicated by B via a lengthwise extending groove 202a provided in the intermediate tray 202 such that the sheet is jogged with the side fences 203 to be aligned. When a sheet is fed into the duplex unit from a main body of an image forming apparatus (not shown), the sheet is reversed with the reverse roller 201 in the direction indicated by an arrow A in FIG. 1(A). Then, the sheet is stacked in the intermediate tray 202 between the pair of side fences 203 and the pressure applied by a pressing roller 204 is at once released as illustrated by a dotted line in FIG. 1(B). The pair of side fences 203 moves to jog a sheet in the direction indicated by an arrow B in FIG. 1(A) such that a distance between the opposed side fences 203 in the sheet width direction is equal to a predetermined sheet width and thereby, a deviation of the sheet positioning, e.g., sheet skew or resulting lateral deviation of sheet positioning, is corrected. A mechanism for driving the pair of side fences 203 is not shown. Thereafter, the reverse roller 201 is switched to be driven in a sheet refeed direction indicated by an arrow C in FIGS. 1(A) and 1(B) and the pressing roller 204 returns to a position to contact the reverse roller 201, so that the sheet is conveyed in the refeed direction by the reversing roller 201 and the pressing roller 204.
However, in the above-described duplex unit, if both sides of a sheet 205 are curled upwardly as indicated by an arrow D in FIG. 2(A), the linear dimension L' from one curled edge to another curled edge of the sheet 205 will be shorter than the sheet width L to which the side fences 203 are adjusted, i.e. L'&lt;L, as illustrated in FIG. 2(B). Accordingly, in this condition, i.e. L'&lt;L, the side fences 203 will not contact the edges of the sheet, even when the fences are driven to jog the sheet. As a result, the curled sheet 205 cannot be positioned adequately in the sheet width direction.
Japanese Laid-Open Patent Publication No.6-51587 of 1994 describes a sheet skew correcting device in a two-sided image forming apparatus in which a sheet stopper is placed ahead of feeding rollers in the sheet refeed direction, so that skew of a sheet is corrected when the sheet abuts the stopper. In this sheet skew correcting device, however, lateral deviation of sheet positioning is not corrected.
Additionally, Japan Patent Publication No. 2692957 of 1997 describes a background image forming apparatus configured to form an image on two sides of a sheet. In a switchback path of a switch back unit to reverse a sheet, a pair of reversible rollers is disposed for reversing a sheet. The switch back unit allows an entrance of a succeeding sheet into the switchback path when a trailing portion of a preceding sheet, which is switched to a reverse direction to be refed, is situated in the switchback path with the pair of reversible rollers being separated from each other. A lateral registration guide is disposed downstream of the reversible rollers for correcting lateral deviation of the sheet.
Japanese Laid-Open Patent Publication No. 8-81105 of 1996 describes a sheet reverse device in which a sheet is reversed by a pair of reversible rollers in a switchback path. The switchback path is Y-shaped and a separation pick, provided at the intersection of the separated paths of the Y-shaped path, switches between the separated paths to guide the sheet.
Japanese Laid-Open Patent Publication No.7-128921 of 1995 describes an optional duplex unit in which a sheet is conveyed to a reverse conveying path which is separated from a sheet conveying path in a main body. The sheet is then reversed by a pair of reversible rollers and adjusted for its sheet position by a sheet position adjusting device.
In Japanese Patent Publication No. 2692957, the switchback path and the lateral registration guide for adjusting sheet positioning are provided at separate positions. Therefore, a space for the switchback path and the lateral registration guide for adjusting sheet positioning is separately necessary, so that the image forming apparatus tends to be a bigger size. Further, because a sheet refeeding path inside the switchback unit is relatively long, a possibility of occurrence of a sheet skew and the resulting lateral deviation of the sheet may be increased. Therefore, a precise sheet positioning adjustment may be required.
In Japanese Laid-Open Patent Publication No. 8-81105, because the separation pick switches between the separated paths of the Y-shaped switch back path, a control operation of the separation pick is additionally required.
In Japanese Laid-Open Patent Publication No. 7-128921, the sheet is reversed and adjusted for its sheet position at separate positions like the sheet described in Japanese Patent Publication No. 2692957. Also, a space for carrying out sheet reversing and sheet positioning adjustment functions is separately necessary. Therefore, the duplex unit needs to be bigger and manufacturing costs may be increased.